I only have a partial answer, but here is what I was able to dig up:
That same Wikipedia page you link contains this:
Tommy Sanford, director of the Commercial Spaceflight Federation, opined that the car and its rocket stage are no more "space junk" than the mundane material usually launched on other test flights. Mass simulators are often deliberately placed in a graveyard orbit or sent on a deep space trajectory, where they are not a hazard.
Clicking through to the source of that quote, here, does indeed have his exact words.
"People could always argue about debris and define it as debris, but when they do those early launches with dummy payload, they make sure they put the dummy payload into a graveyard or inoperable orbit that is not of value to the space community and something that won’t be a threat to future activities in space," said Sanford.
So it would appear that at least some past mass simulators are in a graveyard orbit around either the Earth or the Sun.
Your linked Wikipedia page also has a reference to the boilerplate page, which lists many missions that were considered "boiler plate," but the list lacks information on the fate of the mission hardware. Perhaps more specific research on a case-by-case basis would yield where some of them are. Searching Wikipedia for "mass simulator" redirects to the boilerplate page, so that's no help. I don't have a source confirming what Tommy Sanford said in the above quote, nor anything more specific about it.